Causes of World War II

"The Big Four" made all the major decisions at the Paris Peace Conference (from left to right, David Lloyd George of Britain, Vittorio Emanuele Orlando of Italy, Georges Clemenceau of France, Woodrow Wilson of the U.S.)

By the end of World War I in late 1918, the world's social and geopolitical circumstances had fundamentally and irrevocably changed. The Allies had been victorious, but many of Europe's economies and infrastructures were devastated, including those of the victors. France, along with the other victor countries, was in a desperate situation regarding its economy, security, and morale, and understood that its position in 1918 was "artificial and transitory".[2] Thus, Prime Minister of France Georges Clemenceau worked to gain French security via the Treaty of Versailles, and French security demands, such as reparations, coal payments, and a demilitarized Rhineland, took precedence at the Paris Peace Conference of 1919–1920,[2] which designed the treaty. The war "must be someone's fault – and that's a very natural human reaction" analyzed historian Margaret MacMillan.[3] Germany was charged with the sole responsibility of starting World War I, and the "War Guilt Clause" was the first step towards a satisfying revenge for the victor countries, namely France, against Germany. Ginsberg argues, "France was greatly weakened and, in its weakness and fear of a resurgent Germany, sought to isolate and punish Germany....French revenge would come back to haunt France during the Nazi invasion and occupation twenty years later."[4]

Germany after Versailles

---

  Administered by the League of Nations

  Annexed or transferred to neighboring countries by the treaty, or later via plebiscite and League of Nation action

  Weimar Germany

The two main provisions of the French security agenda were war reparations from Germany in the form of money and coal and a detached German Rhineland. The French government printed excess currency, which created inflation, to compensate for the lack of funds, in addition to borrowing money from the United States. Reparations from Germany were necessary to stabilize the French economy.[5] France also demanded that Germany give France its coal supply from the Ruhr to compensate for the destruction of French coal mines during the war. The French demanded an amount of coal that was a "technical impossibility" for the Germans to pay back.[6] France also insisted on the demilitarization of the German Rhineland in the hope of hindering any possibility of a future German attack. This gave France a physical security barrier between itself and Germany.[7] The inordinate amount of reparations, coal payments, and the principle of a demilitarized Rhineland were largely viewed by the Germans as insulting and unreasonable.

The resulting Treaty of Versailles brought a formal end to the war but was judged by governments on all sides of the conflict: it was neither lenient enough to appease Germany, nor harsh enough to prevent it from becoming a dominant continental power again.[8] The German people largely viewed the treaty as placing the blame, or "war guilt", on Germany and Austria-Hungary and punishing them for their "responsibility", rather than working out an agreement that would assure long-term peace. The treaty imposed harsh monetary reparations as well as demilitarization requirements and territorial dismemberment, and caused mass ethnic resettlement, separating millions of ethnic Germans into neighboring countries.

In the effort to pay war reparations to Britain and France, the Weimar Republic printed trillions of marks, causing extremely high inflation. "No postwar German government believed it could accept such a burden on future generations and survive ...".[5] Paying reparations to the victorious side was a traditional punishment with a long history of use, but in this instance it was the "extreme immoderation" that caused German resentment. Germany did not make its last World War I reparation payment until 3 October 2010,[9] ninety-two years after the end of the war. Germany also fell behind in their coal payments because of a passive resistance movement against the French.[10] In response, the French invaded the Ruhr and occupied it. By this point the majority of Germans were enraged with the French and placed the blame for their humiliation on the Weimar Republic. Adolf Hitler, a leader of the Nazi Party, attempted a coup d'état of the Republic in 1923 known as the Beer Hall Putsch, through which he intended to establish a Greater German Reich.[11] Although this failed, Hitler gained recognition as a national hero among the German population.

During the war, German colonies outside Europe had been annexed by the Allies, and Italy took the southern half of Tyrol after an armistice was agreed upon. The war in the east ended with the defeat and collapse of the Russian Empire, and German troops had occupied large parts of Eastern and Central Europe (with varying degrees of control), establishing various client states such as a kingdom of Poland and the United Baltic Duchy. After the destructive and indecisive Battle of Jutland in 1916 and the mutiny of its sailors in 1917, the Kaiserliche Marine spent most of the war in port, only to be turned over to the Allies and scuttled at surrender by its own officers. Decades later, the lack of an obvious military defeat would be one of the pillars holding together the Dolchstosslegende ("Stab-in-the-back myth"), giving the Nazis another propaganda tool.

Map of territorial changes in Europe after World War I (as of 1923)

The demilitarized Rhineland and additional cutbacks on military also infuriated the Germans. Although it is logical that France would want the Rhineland to be a neutral zone, the fact that France had the power to make that desire happen merely exacerbated German resentment of the French. In addition, the Treaty of Versailles dissolved the German general staff, and possession of navy ships, aircraft, poison gas, tanks, and heavy artillery was made illegal.[7] The humiliation of being bossed around by the victor countries, especially France, and being stripped of their prized military made the Germans resent the Weimar Republic and idolize anyone who stood up to it.[12] Austria also found the treaty unjust, which encouraged Hitler's popularity.

These conditions generated bitter resentment towards the war's victors, who had promised the people of Germany that U.S. President Woodrow Wilson's Fourteen Points would be a guideline for peace; however, the United States played only a minor role in World War I and Wilson could not convince the Allies to agree to adopt his Fourteen Points. Many Germans felt that the German government had agreed to an armistice based on this understanding, while others felt that the German Revolution of 1918–1919 had been orchestrated by the "November criminals" who later assumed office in the new Weimar Republic. The Japanese also started to express resentment against the countries of Western Europe due to the way they were treated during the negotiations of the Treaty of Versailles; their proposition to start bringing the issue of the current racial equality was not put in the final draft and was hardly talked about at the table with nothing given in reward for Japanese participation in the war.[13] The economic and psychological legacies of the First World War persisted well into the interwar period.

The League of Nations was an international peacekeeping organization founded after World War I with the explicit goal of preventing future wars.[14] The League's methods included disarmament, collective security, settling disputes between countries through negotiation diplomacy, and improving global welfare. The diplomatic philosophy behind the League represented a fundamental shift in thought from the preceding century. The old philosophy of "concert of nations", growing out of the Congress of Vienna (1815), saw Europe as a shifting map of alliances among nation-states, creating a balance of power maintained by strong armies and secret agreements. Under the new philosophy, the League would act as a government of governments, with the role of settling disputes between individual nations in an open and legalist forum. Despite President of the United States Woodrow Wilson being the League's chief architect, the United States never joined the League of Nations, which lessened its power and credibility—the addition of a burgeoning industrial and military world power such as the United States might have added more force behind the League's demands and requests.

The official opening of the League of Nations, 15 November 1920

The League lacked an armed force of its own and so depended on member nations to enforce its resolutions, uphold economic sanctions that the League ordered, or provide an army when needed for the League to use. However, individual governments were often very reluctant to do so. After numerous notable successes and some early failures in the 1920s, the League ultimately proved incapable of preventing aggression by the Axis Powers in the 1930s. The reliance upon unanimous decisions, the lack of an independent body of armed forces, and the continued self-interest of its leading members meant that this failure was arguably inevitable.[15][16]

Expansionism is the doctrine of expanding the territorial base (or economic influence) of a country, usually by means of military aggression. Militarism is the principle or policy of maintaining a strong military capability to use aggressively to expand national interests and/or values, with the view that military efficiency is the supreme ideal of a state.[17]

Though the Treaty of Versailles and the League of Nations had sought to stifle expansionist and militarist policies by all actors, the conditions their creators had imposed on the world's new geopolitical situation and the technological circumstances of the era only emboldened the re-emergence of these ideologies during the interwar period. By the early 1930s, a highly-militaristic and aggressive national ideology prevailed in Germany, Japan, and Italy.[18] That attitude fueled advancements in military technology, subversive propaganda, and ultimately territorial expansion as well. It has been observed that the leaders of countries that have been suddenly militarized often feel a need to prove that their armies are formidable, and that was often a contributing factor in the start of conflicts in the interwar period such as the Second Italo-Abyssinian War and the Second Sino-Japanese War.[19]

In Italy, Benito Mussolini sought to create a New Roman Empire, based around the Mediterranean. It invaded Ethiopia as early as 1935, Albania in early 1938, and later Greece. That provoked angry words and an oil embargo from the League of Nations, which failed.

Under the Nazi regime, Germany began its own program of expansion, seeking to restore the "rightful" boundaries of historic Germany. As a prelude toward these goals the Rhineland was remilitarized in March 1936.[20] Also of importance was the idea of a Greater Germany, supporters of which hoped to unite the German people under one nation-state, which included all territories where Germans lived, regardless of whether they happened to be a minority in a particular territory. After the Treaty of Versailles, a unification between Germany and a newly-formed German-Austria, a successor rump state of Austria-Hungary, was prohibited by the Allies, despite the majority of Austrian Germans supporting such a union.

During the period of the Weimar Republic (1919–1933), the Kapp Putsch, an attempted coup d'état against the republican government, was launched by disaffected members of the armed forces. After that event, some of the more radical militarists and nationalists were submerged in grief and despair into the NSDAP, and more moderate elements of militarism declined. The result was an influx of militarily inclined men into the Nazi Party, which, combined with its racial theories, fueled irredentist sentiments and put Germany on a collision course for war with its immediate neighbors.

Japanese march into Zhengyangmen of Beijing after capturing the city in July 1937

In Asia, the Empire of Japan harbored expansionist desires towards Manchuria and the Republic of China. Two contemporaneous factors in Japan contributed both to the growing power of its military and chaos in its ranks in the period leading up to the Second World War. One was the Cabinet Law, which required the Imperial Japanese Army (IJA) and the Imperial Japanese Navy (IJN) to nominate cabinet members before changes could be formed. That essentially gave the military veto power over the formation of any Cabinet in the ostensibly-parliamentary country. The other factor was gekokujō, or institutionalized disobedience by junior officers. It was common for radical junior officers to press their goals to the extent of assassinating their seniors. In 1936, the phenomenon resulted in the February 26 Incident in which junior officers attempted a coup d'état and killed leading members of the Japanese government. In the 1930s, the Great Depression wrecked Japan's economy and gave radical elements within the Japanese military the chance to force the entire military into working towards the conquest of all of Asia. For example, in 1931, the Kwantung Army, a Japanese military force stationed in Manchuria, staged the Mukden Incident, which sparked the invasion of Manchuria and its transformation into the Japanese puppet state of Manchukuo.

Twentieth-century events marked the culmination of a millennium-long process of intermingling between Germans and Slavic peoples. The rise of nationalism in the 19th century made race a centerpiece of political loyalty. The rise of the nation-state had given way to the politics of identity, including pan-Germanism and pan-Slavism. Furthermore, social Darwinist theories framed the coexistence as a "Teuton vs. Slav" struggle for domination, land, and limited resources.[21] Integrating these ideas into their own worldview, the Nazis believed that the Germans, the "Aryan race", were the master race and that the Slavs were inferior.[22]

Japanese occupation of China in 1937

Other than a few coal and iron deposits, and a small oil field on Sakhalin Island, Japan lacked strategic mineral resources. At the start of the 20th century in the Russo-Japanese War, Japan had succeeded in pushing back the East Asian expansion of the Russian Empire in competition for Korea and Manchuria.

Japan's goal after 1931 was economic dominance of most of East Asia, often expressed in Pan-Asian terms of "Asia for the Asians.".[23] Japan was determined to dominate the China market, which the U.S. and other European powers had been dominating. On October 19, 1939, the American Ambassador to Japan, Joseph C. Grew, in a formal address to the America-Japan Society stated:

the new order in East Asia has appeared to include, among other things, depriving Americans of their long established rights in China, and to this the American people are opposed ... American rights and interests in China are being impaired or destroyed by the policies and actions of the Japanese authorities in China.[24]

In 1937 Japan invaded Manchuria and China proper. Under the guise of the Greater East Asia Co-Prosperity Sphere, with slogans such as "Asia for the Asians!" Japan sought to remove the Western powers' influence in China and replace it with Japanese domination.[25][26]

The ongoing conflict in China led to a deepening conflict with the U.S., where public opinion was alarmed by events such as the Nanking Massacre and growing Japanese power. Lengthy talks were held between the U.S. and Japan. When Japan moved into the southern part of French Indochina, President Roosevelt chose to freeze all Japanese assets in the U.S. The intended consequence of this was the halt of oil shipments from the U.S. to Japan, which had supplied 80 percent of Japanese oil imports. The Netherlands and Britain followed suit. With oil reserves that would last only a year and a half during peacetime (much less during wartime), this ABCD line left Japan two choices: comply with the U.S.-led demand to pull out of China, or seize the oilfields in the East Indies from the Netherlands. The Japanese government deemed it unacceptable to retreat from China.[27]

In the late 1980s, the British historian Richard Overy was involved in a historical dispute with Timothy Mason that mostly played out over the pages of the Past and Present journal over the reasons for the outbreak of World War II in 1939. Mason had contended that a "flight into war" had been imposed on Adolf Hitler by a structural economic crisis, which confronted Hitler with the choice of making difficult economic decisions or aggression. Overy argued against Mason's thesis, maintaining that though Germany was faced with economic problems in 1939, the extent of these problems cannot explain aggression against Poland and the reasons for the outbreak of war were due to the choices made by the Nazi leadership.

Mason had argued that the German working-class was always against the Nazi dictatorship; that in the overheated German economy of the late 1930s, German workers could force employers to grant higher wages by leaving for another firm that would grant the desired wage increases; and that such a form of political resistance forced Adolf Hitler to go to war in 1939.[28] Thus, the outbreak of the Second World War was caused by structural economic problems, a "flight into war" imposed by a domestic crisis.[28] The key aspects of the crisis were, according to Mason, a shaky economic recovery that was threatened by a rearmament program overwhelming the economy and in which the regime's nationalist bluster limited its options.[28] In this way, Mason articulated a Primat der Innenpolitik ("primacy of domestic politics") view of World War II's origins through the concept of social imperialism.[29] Mason's Primat der Innenpolitik thesis was in marked contrast to the Primat der Außenpolitik ("primacy of foreign politics"), which is usually used to explain the war.[28] In Mason's opinion, German foreign policy was driven by domestic political considerations, and the launch of World War II in 1939 was best understood as a "barbaric variant of social imperialism".[30]

Mason argued, "Nazi Germany was always bent at some time upon a major war of expansion."[31] However, Mason argued that the timing of such a war was determined by domestic political pressures, especially as relating to a failing economy, and had nothing to do with what Hitler wanted.[31] In Mason's view, from 1936 to 1941, the state of the German economy, not Hitler's 'will' or 'intentions', was the most important determinate on German foreign policy decisions.[32]

Mason argued that the Nazi leaders were so deeply haunted by the 1918 November Revolution that they were most unwilling to see any fall in working-class living standards out of the fear that it might provoke a repetition.[32] According to Mason, by 1939, the "overheating" of the German economy caused by rearmament, the failure of various rearmament plans produced by the shortages of skilled workers, industrial unrest caused by the breakdown of German social policies, and the sharp drop in living standards for the German working class forced Hitler into going to war at a time and place that were not of his choosing.[33]

Mason contended that when faced with the deep socio-economic crisis, the Nazi leadership had decided to embark upon a ruthless "smash and grab" foreign policy of seizing territory in Eastern Europe that could be pitilessly plundered to support living standards in Germany.[34] Mason described German foreign policy as driven by an opportunistic "next victim" syndrome after the Anschluss in which the "promiscuity of aggressive intentions" was nurtured by every successful foreign policy move.[35] Mason's considered the decision to sign the German-Soviet Non-Aggression Pact and to attack Poland despite the risk of a war with Britain and France to be the abandonment by Hitler of his foreign policy program outlined in Mein Kampf that was forced on him by his need to stop a collapsing German economy by seizing territory abroad to be plundered.[33]

For Overy, the problem with Mason's thesis was that it rested on the assumption that in a way not shown by the records of the information was passed on to Hitler about the Germany's economic problems.[36] Overy argued that there was a difference between economic pressures induced by the problems of the Four Year Plan and economic motives to seize raw materials, industry, and foreign reserves of neighboring states as a way of accelerating the Four Year Plan.[37] Overy asserted that Mason downplayed the repressive capacity of the German state as a way of dealing with domestic unhappiness.[36] Finally, Overy argued that there is considerable evidence that Germany felt that it could master the economic problems of rearmament. As one civil servant put it in January 1940, "we have already mastered so many difficulties in the past, that here too, if one or other raw material became extremely scarce, ways and means will always yet be found to get out of a fix."[38]

Adolf Hitler in Bad Godesberg, Germany, 1938

Hitler and his Nazis took full control of Germany in 1933–34 (Machtergreifung), turning it into a dictatorship with a highly hostile outlook toward the Treaty of Versailles and Jews.[39] It solved its unemployment crisis by heavy military spending.[40]

Hitler's diplomatic tactics were to make seemingly reasonable demands, then threatening war if they were not met; concessions were made, he accepted them and moved onto a new demand.[41] When opponents tried to appease him, he accepted the gains that were offered, then went to the next target. That aggressive strategy worked as Germany pulled out of the League of Nations (1933), rejected the Versailles Treaty and began to re-arm with the Anglo-German Naval Agreement (1935), won back the Saar (1935), re-militarized the Rhineland (1936), formed an alliance ("axis") with Mussolini's Italy (1936), sent massive military aid to Franco in the Spanish Civil War (1936–39), seized Austria (1938), took over Czechoslovakia after the British and French appeasement of the Munich Agreement of 1938, formed a peace pact with Stalin's Russia in August 1939 – and finally invaded Poland in September 1939.[42]

In violation of the Treaty of Versailles and the spirit of the Locarno Pact and the Stresa Front, Germany re-militarized the Rhineland on March 7, 1936. It moved German troops into the part of western Germany where, according to the Versailles Treaty, they were not allowed. France could not act because of political instability at the time. King Edward VIII, who thought the Versailles provision was unjust,[43] ordered the government to stand down.[44]

After the Stresa Conference and even as a reaction to the Anglo-German Naval Agreement, the Italian dictator Benito Mussolini attempted to expand the Italian Empire in Africa by invading the Ethiopian Empire, also known as the Abyssinian Empire. The League of Nations declared Italy to be the aggressor and imposed sanctions on oil sales that proved ineffective. Italy annexed Ethiopia on May 7 and merged Ethiopia, Eritrea, and Somaliland into a single colony, known as Italian East Africa. On June 30, 1936, Ethiopian Emperor Haile Selassie gave a stirring speech before the League of Nations denouncing Italy's actions and criticizing the world community for standing by. He warned, "It is us today. It will be you tomorrow." As a result of the League's condemnation of Italy, Mussolini declared the country's withdrawal from the organization.[45]

Francisco Franco and Heinrich Himmler in Madrid, Spain, 1940

Between 1936 and 1939, Germany and Italy lent support to the Nationalists led by general Francisco Franco in Spain, while the Soviet Union supported the existing democratically elected government, the Spanish Republic, led by Manuel Azaña. Both sides experimented with new weapons and tactics. The League of Nations was never involved, and the major powers of the League remained neutral and tried (with little success) to stop arms shipments into Spain. The Nationalists eventually defeated the Republicans in 1939.[46]

Spain negotiated with joining the Axis but remained neutral during World War II, and did business with both sides. It also sent a volunteer unit to help the Germans against the USSR. Whilst it was considered in the 1940s and 1950s to be a prelude to World War II and it prefigured the war to some extent (as it changed it into an antifascists contest after 1941), it bore no resemblance to the war that started in 1939 and had no major role in causing it.[47][48]

In 1931 Japan took advantage of China's weakness in the Warlord Era and fabricated the Mukden Incident in 1931 to set up the puppet state of Manchukuo in Manchuria, with Puyi, who had been the last emperor of China, as its emperor. In 1937 the Marco Polo Bridge Incident triggered the Second Sino-Japanese War.

The invasion was launched by the bombing of many cities such as Shanghai, Nanjing and Guangzhou. The latest, which began on 22 and 23 September 1937, called forth widespread protests culminating in a resolution by the Far Eastern Advisory Committee of the League of Nations. The Imperial Japanese Army captured the Chinese capital city of Nanjing, and committed war crimes in the Nanjing massacre. The war tied down large numbers of Chinese soldiers, so Japan set up three different Chinese puppet states to enlist some Chinese support.[49]

Cheering crowds greet the Nazis in Innsbruck

The Anschluss was the 1938 annexation by threat of force of Austria into Germany. Historically, the Pan-Germanism idea of creating a Greater Germany to include all ethnic Germans into one nation-state was popular for Germans in both Austria and Germany.

Language groups of Central Europe in 1937.

One of the Nazi party's points was "We demand the unification of all Germans in the Greater Germany on the basis of the people's right to self-determination."

The Stresa Front of 1935 between Britain, France and Italy had guaranteed the independence of Austria, but after the creation of the Rome-Berlin Axis Mussolini was much less interested in upholding its independence.

The Austrian government resisted as long as possible, but had no outside support and finally gave in to Hitler's fiery demands. No fighting occurred as most Austrians were enthusiastic, and Austria was fully absorbed as part of Germany. Outside powers did nothing. Italy had little reason for continued opposition to Germany, and was if anything drawn in closer to the Nazis.[50][51]

The Sudetenland was a predominantly-German region in Czechoslovakia along the border with Germany. It had more than three million ethnic Germans, who comprised almost a quarter of the population of Czechoslovakia. In the Treaty of Versailles it was given to the Czechoslovakia against the wishes of most of the local population. The decision to disregard their right to self-determination was based on France's intent to weaken Germany. Much of Sudetenland was industrialized.[52]

British Prime Minister Neville Chamberlain and Hitler at a meeting in Germany on 24 September 1938, where Hitler demanded the annexation of Czechoslovak border areas without delay.

Czechoslovakia had a modern army of 38 divisions, backed by a well-noted armament industry (Škoda) as well as military alliances with France and the Soviet Union. However, its defensive strategy against Germany was based on the mountains of the Sudetenland.

Hitler pressed for the Sudetenland's incorporation into Germany and supported German separatist groups within the region. Alleged Czechoslovak brutality and persecution under Prague helped to stir up nationalist tendencies, as did the Nazi press. After the Anschluss, all German parties (except the German Social-Democratic party) merged with the Sudeten German Party (SdP). Paramilitary activity and extremist violence peaked during the period, and the Czechoslovakian government declared martial law in parts of the Sudetenland to maintain order. That only complicated the situation, especially since Slovak nationalism was rising, out of suspicion towards Prague and Nazi encouragement. Citing the need to protect the Germans in Czechoslovakia, Germany requested the immediate annexation of the Sudetenland.

In the Munich Agreement on September 30, 1938, the British, French, and Italian prime ministers appeased Hitler by giving him what he wanted in the hope he would not want any more. The conferring powers allowed Germany to move troops into the region and incorporate it into the Reich "for the sake of peace." In exchange, Hitler gave his word that Germany would make no further territorial claims in Europe.[53] Czechoslovakia was not allowed to participate in the conference. When the French and British negotiators informed the Czechoslovak representatives about the agreement and that if Czechoslovakia would not accept it, France and Britain would consider Czechoslovakia to be responsible for war, Czechoslovak President Edvard Beneš capitulated, and Germany took the Sudetenland unopposed.[54]

Chamberlain's policies have been the subject of intense debate for more than 70 years among academics, politicians, and diplomats. The historians' assessments have ranged from condemnation for allowing Hitler's Germany to grow too strong to the judgment that Germany was so strong that it might well win a war and that the postponement of a showdown was in their country's best interests. [55]

All territories taken from Czechoslovakia by its neighbours in October 1938 ("Munich Dictate") and March 1939

In March 1939, breaking the Munich Agreement, German troops invaded Prague, and with the Slovaks declaring independence, Czechoslovakia disappeared as a country. The entire ordeal ended the French and British policy of appeasement.

After the German occupation of Czechoslovakia, Mussolini feared for Italy becoming a second-rate member of the Axis. Rome delivered Tirana an ultimatum on March 25, 1939 by demanding the accession to Italy's occupation of Albania. Albania's King Zog refused to accept money in exchange for allowing a full Italian takeover and colonization of Albania.

On April 7, 1939, Italian troops invaded Albania, which was occupied after a three-day campaign, with minimal resistance offered by Albanian forces.

In 1939, the Japanese attacked west from Manchuria into the Mongolian People's Republic after the earlier 1938 Battle of Lake Khasan. They were decisively beaten by Soviet units, under General Georgy Zhukov. After the battle, the Soviet Union and Japan were at peace until 1945. Japan looked south to expand its empire, leading to conflict with the United States over the Philippines and control of shipping lanes to the Dutch East Indies. The Soviet Union focused on its western border but left 1 million to 1.5 million troops to guard its border with Japan.

The Polish Corridor and the Free City of Danzig

After the final fate of Czechoslovakia proved that the Führer's word could not be trusted, Britain and France decided on a change of strategy. They decided any further unilateral German expansion would be met by force. The natural next target for the Third Reich's further expansion was Poland, whose access to the Baltic sea had been carved out of West Prussia by the Versailles treaty, making East Prussia an exclave. The main port of the area, Danzig, had been made a free city-state under Polish influence guaranteed by the League of Nations, a stark reminder to German nationalists of the Napoleonic free city established after the French emperor's crushing victory over Prussia in 1807.

After taking power, the Nazi government made efforts to establish friendly relations with Poland, resulting in the signing of the ten-year German–Polish Non-Aggression Pact with the Piłsudski regime in 1934. In 1938, Poland participated in the dismemberment of Czechoslovakia by annexing Zaolzie. In 1939, Hitler claimed extraterritoriality for the Reichsautobahn Berlin-Königsberg and a change in Danzig's status, in exchange for promises of territory in Poland's neighbours and a 25-year extension of the non-aggression pact. Poland refused, fearing losing de facto access to the sea, subjugation as a German satellite state or client state, and future further German demands.[56][57] In August 1939, Hitler delivered an ultimatum to Poland on Danzig's status.

In March 1939, Britain and France guaranteed the independence of Poland. Hitler's claims in the summer of 1939 on Danzig and the Polish Corridor provoked yet another international crisis. On August 25, Britain signed the Polish-British Common Defence Pact.

Nominally, the Molotov–Ribbentrop Pact was a non-aggression treaty between Germany and the Soviet Union. It was signed in Moscow on August 23, 1939, by the Soviet foreign minister Vyacheslav Molotov and the German foreign minister Joachim von Ribbentrop.

In 1939, neither Germany nor the Soviet Union was ready to go to war with each other. The Soviet Union had lost territory to Poland in 1920. Although officially labeled a "non-aggression treaty," the pact included a secret protocol in which the independent countries of Finland, Estonia, Latvia, Lithuania, Poland and Romania were divided into spheres of interest of the parties. The secret protocol explicitly assumed "territorial and political rearrangements" in those countries' areas.

All of those mentioned countries were invaded, occupied, or forced to cede part of their territory by the Soviet Union, Germany, or both.

Germany invaded Poland on 1 September 1939 which directly led to the Anglo-French declaration of war on Germany on 3 Sept. The Soviet Union joined Germany's invasion of Poland on 17 September.

Between 1919 and 1939 Poland pursued a policy of balancing between the Soviet Union and Nazi Germany, seeking non-aggression treaties with both.[58] In early 1939, Germany demanded that Poland should join the Anti-Comintern Pact as a satellite state of Germany.[59] Poland, fearing a loss of independence, refused, and Hitler told his generals on 23 May 1939 that the reason for invading Poland was not Danzig: "Danzig is not the issue at stake. It's a matter of extending our living space in the East..."[60] To deter Hitler, Britain and France announced that an invasion would mean war and tried to convince the Soviet Union to join in this deterrence. The USSR, however, gained control of the Baltic states and parts of Poland by allying with Germany, which it did through the secret Molotov–Ribbentrop Pact in August 1939. London's attempt at deterrence failed, but Hitler did not expect a wider war. Germany invaded Poland on September 1, 1939, and rejected the British and French demands that it withdraw, resulting in their declaration of war on September 3, 1939, in accordance with the defense treaties with Poland that they had signed and publicly announced.[61][62]

Germany attacked the Soviet Union in June 1941. Hitler believed that the Soviet Union could be defeated in a fast-paced and relentless assault that capitalized on the Soviet Union's ill-prepared state, and hoped that success there would bring Britain to the negotiation table, ending the war altogether.

The US government and the American public in general had been supportive of China, condemning the colonialist policies of the European powers and Japan, and promoting the so-called Open Door Policy. Many Americans viewed the Japanese as an aggressive and/or inferior race. The Nationalist government of Chiang Kai-shek held friendly relations with the United States, which opposed Japan's invasion of China in 1937 and considered it a violation of international law and of the sovereignty of the Republic of China. It offered the Nationalist government diplomatic, economic, and military assistance during its war against Japan. Diplomatic friction between the United States and Japan manifested itself in events like the Panay incident in 1937 and the Allison incident in 1938.

Japanese troops entering Saigon

Reacting to Japanese pressure on French authorities of French Indochina to stop trade with China, the U.S. began restricting trade with Japan in July 1940. The end of all oil shipments in 1941 was decisive since the US, Britain, and the Netherlands provided almost all of Japan's oil.[63] In September 1940, the Japanese invaded Vichy French Indochina and occupied Tonkin to prevent China from importing arms and fuel through French Indochina along the Sino-Vietnamese Railway from the port of Haiphong through Hanoi to Kunming in Yunnan.[64] The US decided that the Japanese had now gone too far and decided to force a rollback of its gains.[65] In 1940 and 1941, the US and China decided to organize a volunteer squadron of American planes and pilots to attack Japan from Chinese bases. Known as the Flying Tigers, the unit was commanded by Claire Lee Chennault. Their first combat came two weeks after the attack on Pearl Harbor.[66]

Taking advantage of the situation, Thailand launched the Franco-Thai War in October 1940. Japan stepped in as a mediator in the war in May 1941, allowing its ally to occupy the bordering provinces in Cambodia and Laos. In July 1941, as Operation Barbarossa]] had [[effectively neutralized the Soviet threat, the faction of the Japanese military junta supporting the "Southern Strategy" pushed through the occupation of the rest of French Indochina.

The United States reacted by seeking to bring the Japanese war effort to a complete halt by imposing a full embargo on all trade between the United States to Japan on August 18, 1941, demanding a Japanese withdrawal of all troops from China and Indochina. Japan was dependent on the United States for 80% of its oil, resulting in an economic and military crisis for Japan that could not continue its war effort with China without access to petroleum and oil products.[67]

Attack on Pearl Harbor, December 1941

On 7 December 1941, without any prior declaration of war,[68] the Imperial Japanese Navy attacked Pearl Harbor with the aim of destroying the main American battle fleet at anchor. At the same time, other Japanese forces attacked the American-held Philippines and the British Empire in Malaya, Singapore, and Hong Kong. The following day, an official Japanese declaration of war on the United States and the British Empire was printed on the front page of all Japanese newspapers' evening editions.[69] International time differences caused the announcement to take place between midnight and 3 a.m. on 8 December in North America and at about 8 a.m. on 8 December in the United Kingdom.

Canada declared war on Japan on the evening of 7 December; a royal proclamation affirmed the declaration the next day.[70] The United Kingdom declared war on Japan on the morning of 8 December and specifically identified the attacks on Malaya, Singapore, and Hong Kong as the cause but omitted any mention of Pearl Harbor.[71] The United States declared war upon Japan on the afternoon of 8 December, some nine hours after the United Kingdom, and identified only "unprovoked acts of war against the Government and the people of the United States of America" as the cause.[72]

Four days later, the US was brought into the European war when on December 11, 1941, Nazi Germany and Fascist Italy declared war on the United States. Hitler chose to declare that the Tripartite Pact required Germany to follow Japan's declaration of war although American destroyers escorting convoys and German U-boats had already been de facto at war in the Battle of the Atlantic. That declaration of war effectively ended isolationist sentiment in the US, which immediately reciprocated and so formally entered the war in Europe.[73]